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Speleothems As Environmental Recorders a Study of Holocene Speleothems and Their Growth Environments in Sweden

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Speleothems As Environmental Recorders a Study of Holocene Speleothems and Their Growth Environments in Sweden Speleothems as environmental recorders A study of Holocene speleothems and their growth environments in Sweden Hanna S. Sundqvist Thesis Contents This doctoral thesis consists of a summary and four appended papers which will be referred to in the text by their Roman numerals. List of papers I Sundqvist, H. S., Seibert, J. and Holmgren, K. Understanding conditions behind speleothem formation in Korallgrottan, northwestern Sweden. Submitted to Journal of Hydrology. II Sundqvist, H. S., Holmgren, K. and Lauritzen, S.-E., 2007. Stable isotope variations in sta- lagmites from northwestern Sweden document changes in temperature and vegetation, during early Holocene. The Holocene, 17(1). III Sundqvist, H. S., Holmgren, K., Moberg, A., Spötl, C. and Mangini, A. Stable oxygen isotopes in a stalagmite from Jämtland, NW Sweden, record large temperature variations over the last 4000 years. Submitted to Geophysical Research Letters. IV Sundqvist, H. S., Baker, A. and Holmgren, K. 2005. Luminescence in fast growing stalagmites from Uppsala, Sweden. Geografiska Annaler, 87 A (4): 539-548. The co-authorship of Papers I-IV I have planned and designed the study, carried out the field work, performed most of the analyses and led all the paper writing. In paper I Jan Seibert did the HBV modelling. In paper II Stein Erik Lauritzen contributed with data from a Norwegian cave and the samples were dated at his labora- tory. Christoph Spötl and Augusto Mangini contributed to paper III by being responsible for the stable isotope and dating laboratories used. Anders Moberg contributed to the scientific discussion and to the choice of statistical treatment of data in paper III. In paper IV, I and Andy Baker jointly performed the luminescence analysis. The thesis project was initiated by Karin Holmgren who made the first visits and did the first sampling in them. She has also participated several times in field and contributed to all papers through scientific discussions and improvements to the text. 1 Hanna S. Sundqvist Introduction Project objectives To understand human impact on climate today we need The overall aim with this PhD thesis is been to contrib- to reconstruct and understand how climate has varied ute highly resolved regional palaeoenvironmental infor- back in time, well before the industrial revolution. mation for the Holocene time period through studies of Knowledge about natural climate variability is also speleothems and their growth environments at different essential in studies of how human societies have been locations in Sweden. The specific objectives were: affected by and have adapted to climate change in the past. Climate change can be studied in a number of 1. To determine if speleothems from three Swedish natural archives. Speleothems, or cave drip stones are sites are suitable as palaeoclimatic archives. one of those archives which, like ice cores, tree rings, peat and lake sediments, can be used for reconstruct- 2. To obtain an understanding of factors that control ing past climate change. Carbonate speleothems are and affect the growth and properties of the spele- secondary deposits of calcium carbonate, chemically othems at these specific sites. precipitated in caves from carbonate seepage water. They are deposited either through degassing of car- 3. To examine if stable isotopes, luminescence and bon dioxide or through evaporation (Schwarcz 1986). trace elements in the speleothems can be used as Speleothems are well suited for uranium-series dat- proxy indicators of past environmental change. ing, producing ages directly in calendar years (Smart 1991, Ivanovich and Harmon 1992) and the mecha- 4. To provide palaeoclimatic data for the Holocene nisms controlling speleothem growth are sensitive to time period and compare it with other proxy data external, often climatically driven, processes. There- from the region. fore, many variables that can be measured in spele- othems, such as stable carbon and oxygen isotopes, laminations and trace element composition, may serve as climatic proxies (Lauritzen and Lundberg 1999a). Speleothems as climatic archives Even though the advantages are acknowledged, prob- lems still exist in using speleothems as palaeoclimatic Speleothems often hold a remarkable archive of data, archives (Smart and Richards 2003). Like other cli- describing local and global climatic and environmen- mate archives, speleothems are individuals. Each spe- tal conditions for the period of time in which they grew leothem is unique with its own response to external (Henderson 2006). The first palaeoclimatic studies of spe- processes; therefore ideally more than one sample leothems were conducted almost forty years ago (Hendy from each cave should be used. However, this ambi- and Wilson 1968, Thompson et al. 1974). The most im- tion can conflict with issues concerning cave conser- portant analytical development since the first studies is vation. Furthermore, karst processes are characterised the improvements in the technique of dating speleothems. by non-linear and threshold responses. To understand Unlike other continental archives speleothems are not these, laboratory and field experiments are needed to commonly dated with the radiocarbon method owing to determine the specific factors affecting speleothem the variable proportion of “dead” carbon from the bedrock deposition at different sites. (Genty et al. 2001). Instead speleothems are almost ideal In Sweden, caves with speleothems are found in for U-series dating. Uranium (238U, 235U, 234U) is bound Lummelundagrottan, developed in Silurian limestone, in silicate and oxide minerals in rocks and is co-precipi- on the island Gotland, and in caves developed in tated with the calcium carbonate forming the speleothem. Precambrian limestone or marble in the Caledonian It decays into daughter isotopes (230Th, 232Th), and as tho- mountain range in northern Sweden (Engh 1981). The rium is insoluble in water, is therefore absent at the time speleothems in Lummelundagrottan have been thor- of deposition. The production of 230Th from disintegra- oughly described by Engh (1981). A study of growth tion of 234U thus serves as a measure of time (Smart 1991, layers in a stalagmite from Lummelundagrottan was Ivanovich et al. 1992). However, contamination of the performed by Carlsson (1998). While the speleothems daughter isotope 230Th may occur as a result of co-pre- in Lummelundagrottan have proved to be difficult to cipitation of 232Th attached to clay particles present in the date, because of low uranium content in the Silurian cave drip water. However, it is possible to corrected for limestone, the speleothems from the mountain caves this. Another problem is that results from different dating contain enough uranium for precise uranium series labs are not strictly comparable because of the different dating, using available high resolution dating tech- spike calibration and correction methods (McDermott et niques. al. 2006). The dating techniques are straightforward and 2 Speleothems as palaeoenvironmental recorders have been described thoroughly elsewhere (Smart 1991, Oxygen isotopes Ivanovich 1992). The advances in Thermal Ionisation The stable oxygen isotope composition of speleothem Mass Spectrometry (TIMS) have increased precision and calcite formed in isotopic equilibrium is related to the reduced the sample size, making it possible to achieve isotopic composition of the drip water and the cave chronological precision as good as 0.3-0.6 % (Lauritzen temperature. Studies have shown that the δ18O composi- and Lundberg 1999b). The recently introduced high-res- tion of cave drip water can be constant throughout the olution magnetic-sector field ICP-MS (inductively cou- year, and be approximately equal to the composition pled plasma mass spectrometry) techniques require even of the mean annual value of the outside precipitation smaller sample sizes than the TIMS-technique (Halliday (Schwarcz et al. 1976, Younge et al. 1985) and that the et al. 1998, Shen et al. 2002). In this thesis, the main deep cave temperature is close to the mean annual tem- speleothem variables studied are stable isotopes and lu- perature outside the cave (Wigley and Brown 1976). minescent laminations and therefore a short summary of The δ18O composition of precipitation is a consequence the theory behind these proxies is given. of several factors such as latitude, altitude, distance from sea, amount of precipitation and air temperature Stable isotopes reflecting the mass-fraction of moisture precipitated 18 When there is slow degassing of CO2, stable temperature from clouds. The mean annual δ O composition of pre- and no evaporation, calcium carbonate can be precipi- cipitation decreases systematically across Europe with tated in isotopic equilibrium with the parent drip water increasing distance from the North Atlantic Ocean (Ro- (Hendy 1971). This means that the partitioning of light zanski et al. 1993). Under present-day conditions, the and heavy isotopes between the aqueous and solid phase temperature dependence in rainfall is about 0.59 °± 0.09 is only a function of cave air temperature (Lauritzen and ‰°C-1 for European sites (Rozanski et al.1993). Since Lundberg 1999b). The calcite-water fractionation is – this exceeds the –0.24 ‰°C-1 calcite-water fractiona- 0.24 ‰ °C-1 (O´Neill et al. 1969). Characteristic features tion, a net shift to heavier O isotopes with higher tem- of equilibrium deposits are according to Hendy (1971): peratures is expected (i.e. a positive correlation between δ18O in the calcite and temperature)(Tab.1).
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